1. Field of the Invention
This invention relates to electrical contact elements which are movable into circuit making and breaking relation with respect to each other and to electrical contactors, relays, switches, or the like embodying such contact elements, and to a composite energy absorbing and contact pressure applying device for use with such contact elements.
2. Description of the Prior Art
In many types of electrical contactors or electrical circuit makers and breakers in commercial use, the movement of the movable contact into engagement with the stationary or fixed contact to close the electrical circuit through the contacts involves an undesirable bouncing action of the movable contact relative to the fixed contact. In most such electrical contactors or circuit makers and breakers a metal biasing spring is used to hold the movable contact under pressure against the fixed contact in the closed position of the contact to reduce contact resistance and consequent I.sup.2 R loss through the closed contacts. While the biasing spring which biases the movable contact toward the fixed contact performs an important function once the contacts have finally closed, the resiliency of the biasing springs used in the prior art contribute significantly to undesirable contact bounce during the initial closing period of the contacts.
Bouncing of the movable contact upon closure of the movable contact into engagement with the fixed contact is very undesirable due not only to the mechanical wear which occurs on the contacts due to the contact bouncing, but also due to the damaging effects of the arcing which occurs during the bouncing period. Such arching causing erosion of the contact making surfaces and in extreme cases due to the extremely high temperature of the arc (a cathode spot can reach temperatures of 3500.degree. Kelvin), may cause welding of the movable contact and fixed contact to each other to prevent reopening of the movable contact when desired. Contact welding can cause failure of associated equipment resulting in possible property and personnel damage.
When a typical electrical contactor is used to close the electrical circuit to certain types of electrical loads, the curve of current vs. time may initially rise very steeply from zero to some value such as 2600 amperes, for example, in a very short time interval such as 13 milliseconds, for example. (200 amperes per millisecond.)
If the rate of rise of the magnitude of the electrical current to the load device during the closing operation of the movable contact relative to the fixed contact is some typical value such as 200 amperes per millisecond, it can be seen that if the period during which the movable contact bounces relative to the fixed contact lasts as long as 0.013 second (13 milliseconds) which is a typical condition in accordance with prior art contact devices, then the arc current flow between the bouncing movable contact and the fixed contact will have reached a value such as 2600 amperes by the end of the 13 millisecond interval of contact bouncing. This extremely high arc current during the prolonged period of bounce will accelerate erosion of the mating contacts and will cause possible welding of the contacts as previously explained.
On the other hand, if the period during which bouncing of the movable contact relative to the fixed contact occurs is reduced to a much lower time duration or interval, such as 0.002 second (2 milliseconds), to correspondingly reduce the arcing period between the contacts in accordance with the present invention, as well as reducing the maximum amplitude of arc current reached during the period of bouncing, then it can be seen that erosion of the contacts due to arcing during the closing period, and also the possibility of welding of the movable and fixed contacts to each other can be substantially reduced as compared to the prior art. In addition, the mechanical wear on the contacts due to the impact of bouncing is significantly reduced due to the significant reduction of the duration of the contact bouncing.
It can be said without fear of contradiction that contact life is inversely proportional to contact bounce and contact material loss is directly proportional to contact bounce.